During the past several decades, coronary heart disease has come to occupy the first position among the causes of death in the developed areas of the world. In the United States, for example, this disease is responsible for over one-half million deaths yearly. And of this number, more than half occur suddenly, outside the hospital, and therefore before the patient is able to obtain the necessary medical assistance. Although the precise cause of sudden death in coronary heart disease has not yet been entirely clarified, the available evidence permits the medical field to ascribe death in the majority of these cases to grave disturbances in cardiac electrical activity culminating in ventricular fibrillation.
The frustrating but related problem is the present inability to deal effectively with lethal and non-lethal arrhythmias outside of a hospital setting. Within the hospital environment, recent experience has clearly demonstrated that ventricular fibrillation and its frequent precursor, ventricular tachycardia, are reversible phenomena when prompt defibrillation of the heart is instituted. Under such circumstances, cardiac function can frequently be restored to normal without the patient suffering from residual disability. Unfortunately, however, the state of the art makes defibrillation very much dependent upon a highly specialized medical environment, thus limiting such treatment to elaborately equipped modern hospitals.
There is no question that a great need exists for a defibrillator which could be carried by those who are prone to having one of the many life threatening arrhythmias generally discussed above. Thus, in some patients having coronary heart disease, a fatal outcome from ventricular tachycardia or ventricular fibrillation could be avoided, even in the absence of immediate medical assistance. The first step, of course, is the detection of those prone to suffering from cardiac malfunctions leading to ventricular tachycardia or ventricular fibrillation.
While it is not possible to predict with unerring certainty which patients suffering from coronary heart disease will be the victims of sudden death, several high risk groups of patients can be recognized. For example, patients who have experienced myocardial infarction, even though they may be surviving in good health, run a substantial risk of dying suddenly, a risk several times greater than that associated with the general population. Further, if patients with myocardial infarction have a history of serious ventricular arrhythmias and/or of cardiac arrest, or if evidence of persistent myocardial irritability is present, it may logically be assumed that the risk of sudden death is increased substantially. Patients like those described above would greatly benefit from an automatic, standby or demand defibrillator.
Also, such an automatic defibrillator would be an asset to those hospitalized patients who have suffered myocardial infarction both in the coronary care unit and those who have been discharged from the well-equipped coronary care unit. Under such circumstances, the defibrillator could be utilized temporarily for the remainder of the expected hospital stay; or the automatic defibrillator could be permanently implanted for use both in the hospital and after discharge.
Another recognizable class of patients particularly in need of an automatic defibrillator is the class composed of those who have not shown prior histories of myocardial infarction but who show severe symptoms of coronary heart disease, such as ventricular arrhythmias resistant to medical treatment or angina pectoris.
From the brief discussion above, there should be little doubt that the possible applications for an automatic defibrillator are numerous. Such automatic standby defibrillators have been developed and are described in U.S. Pat. No. Re. 27,757 entitled STANDBY DEFIBRILLATOR AND METHOD OF OPERATION and U.S. Pat. No. Re. 27,652 entitled ELECTRONIC STANDBY DEFIBRILLATOR.
The automatic standby defibrillators described in the above-identified patents employ a sensing probe carrying one electrode which is positioned in the right ventricle of the heart. A second electrode, separate from and unconnected to the sensing probe, is generally a flat plate either placed on the outer surface of the chest, sutured under the skin of the anterior chest wall or applied directly to the ventricular myocardium. As a result, these systems require much more surgery and, depending on the position of the second electrode, may ultimately be only partially implanted. In addition, such systems require a capacitor of sufficient size and capacity to store approximately several hundreds of watt-seconds, which is the necessary energy required in those systems to produce defibrillation. Therefore, it is evident that an automatic standby or even a manually initiated defibrillating system which requires a much lower energy level, thereby being much smaller and compact in size and which may be totally and completely implanted within the patient, would be highly desirable and would represent a great improvement.
Another drawback of the prior art relates to the possibility of developing ventricular fibrillation in the course of correcting, for example, atrial fibrillation. In the prior art, the heart is often brought out of artial fibrillation by delivering an electrical shock across the chest by means of precordial electrodes, or paddles. With such an arrangement, it is necessary to develop a large electrical potential across the paddles, but impossible to centralize the potential across the atria. Accordingly, it is essential to very carefully time the pulses delivered to bring a malfunctioning heart out of atrial fibrillation, or the large shock across the ventricles may induce ventricular fibrillation. This is yet another indication of the need for a low-energy intravascular electrode system capable of accurately delivering electrical pulses to localized areas of the heart.
Two other disadvantages of the prior art high-energy cardioverting systems, and the need for improvement, should be noted. First, because of the extreme pain which could be experienced by a patient undergoing high-energy cardioversion, it is the practice to administer a general anesthetic before an attempted cardioversion. With a low-energy intravascular electrode system, the use of a general anesthetic can be avoided.
Secondly, the application of high-energy electrical shocks across the chest of a patient carries with it substantial risk of cardiac damage. Again, by way of a low-energy intravascular electrode system capable of localizing defibrillating electrical shocks, the potential risk of injuring the myocardium is considerably decreased.
In this regard, efforts have been made to experiment with defibrillators other than that described above. For one, see Hopps et al., "Electrical Treatment of Cardiac Arrest: A Cardiac Stimulator-Defibrillator," Surgery, Vol. 36, No. 4 (Oct. 1954), at pages 833-849. There, the researchers attempted to bring dogs out of ventricular fibrillation by using an intracardiac electrode carrying two very closely spaced electrodes. The authors concluded that shocks applied through an intracardiac catheter were not effective in cardiac defibrillation. A second example can be seen in recently issued U.S. Pat. No. 3,738,370. There, a single catheter carries a pair of electrodes spaced together so closely as to allow placement of both electrodes in the atrium. Though the patentee alleges effective cardioversion, the energy levels are higher than are actually necessary. The electrode spacing and location are seen to be the reasons. Therefore, to date, the medical field is without a simple, low-energy cardioverting device using a single intravascular electrode catheter capable of treating a wide range of atrial and ventricular arrythmias.